Control of CDMA signal integration

ABSTRACT

A method for controlling integration for a CDMA signal and a receiver implementing the same. In accordance with the present invention, non-coherent and even coherent integration periods for a received signal are dynamically and adaptively controlled depending upon the condition of the received signal. The integration period can be very short when signal strength is strong and no blocking exists; while it can be extended to be longer when the signal strength is weak or there is a blocking. Therefore, it is possible to keep locking even under bad circumstances. In addition, the fix time can be shortened when the signal strength is very strong.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to GNSS navigation, more particularly, tocontrol of coherent and non-coherent integrations of a receiver for CDMAsignals.

BACKGROUND OF THE INVENTION

Code Division Multiple Access (CDMA) has been widely used in satellitecommunication field such as GNSS. Civic GPS (Global Positioning System)application is rapidly developed. To achieve excellent GPS navigation,sufficiently available satellite number and qualified signal measurementaccuracy are basic requirements to be satisfied. However, suchconditions are hardly satisfied in deep urban circumstances.

A GPS receiver suffers problem when receiving signals in urban. Forexample, signal power level may abruptly and rapidly change since thereceiver, which may be set up in a vehicle, is moving, or the signal isblocked by buildings.

The receiver uses coherent integration and non-coherent integration toobtain sufficient signal power. Conventionally, a coherent integrationperiod is fixed and is ranged from 1 ms to 20 ms, in general. Inaddition, a non-coherent number is also set in advance. If the coherentintegration period is predetermined to be 20 ms, the receiver integratesthe receiver power for 20 ms. The 20 ms coherent integration is stored,and another 20 ms coherent integration is calculated. The 20 ms coherentintegrations are accumulated. The non-coherent number of coherentintegrations are accumulated. This is called non-coherent integration. Anon-coherent integration period is predetermined since the non-coherentnumber is fixed. For example, the non-coherent integration period ispredetermined as 2 seconds. Then, the 20 ms coherent integrations areaccumulated until a 2-second non-coherent integration is obtained. Thenon-coherent integrated signal power is checked to see if it issufficiently high. However, in bad circumstances, such as urban, thesignal may be sometimes very weak or even be blocked. It will take asignificantly long time to obtain sufficient signal power level by usingthe prior art scheme. For example, when a receiver receives a satellitesignal in urban environment, the power of the satellite signal may bechanged dramatically. The strong signal power may suddenly become weak.Under such a circumstance, the receiver is likely to lose lock for thesatellite if the integration time is not long enough. Therefore, it isnecessary to use a long integration interval. However, the weak signalpower may also suddenly become strong. If the long integration timeinterval is still used, the response of the receiver to the satellitesignal will become slow and blunt. To overcome such a problem, theintegration period needs to be more effectively and adaptivelycontrolled.

SUMMARY OF THE INVENTION

The present invention is to provide a method for controlling integrationfor a CDMA signal. By using the method of the present invention,integration period for the signal can be effectively and adaptivelycontrolled. Therefore, it is possible to keep locking the signal evenwhen the signal strength is weak or there is blocking. In addition, thefix time can be effectively shortened.

The method comprises steps of executing coherent integration for acoherent integration period and storing the coherent integration resultinto a first memory; calculating a signal power from the coherentintegration result; accumulating the signal power of each coherentintegration into a second memory as a non-coherent integration comparingthe accumulated signal power with a threshold; and repeating the abovesteps if the accumulated signal power has not exceeded the thresholdwhile stopping accumulating if the accumulated signal power exceeds thethreshold.

In the case that the coherent period is set to be long, the coherentperiod can be divided into a plurality of sub-units. The coherentintegration result is calculated to obtain an updated signal power byevery sub-unit, and the signal power is compared with the threshold, thesignal power is used in further processing if it exceeds the threshold.

The present invention is further to provide a receiver for receivingCDMA signals. The integration period for the signal received by thereceiver can be effectively and adaptively controlled. Therefore, it ispossible to keep locking the signal even when the signal strength isweak or there is blocking. In addition, the fix time can be effectivelyshortened.

The receiver comprises an antenna for receiving a signal; a correlatorfor executing correlation to the signal; a coherent integration blockexecuting a coherent integration to the signal for a coherentintegration period; a magnitude calculator calculating a signal powerfrom a result of the coherent integration of the coherent integrationblock; a non-coherent integration block accumulating the signal power ofeach coherent integration as a non-coherent integration; a comparatorcomparing the signal power with a threshold whenever signal power of acoherent integration is accumulated to the non-coherent block; and acontroller stopping the non-coherent integration and passing the signalto further process when the signal power exceeds the threshold.

In the case that the coherent period is set to be long, the coherentintegration period is divided into a plurality of sub-units, themagnitude calculator calculates the coherent integration result andupdates the signal power by each sub-unit for the first coherentintegration period, and the comparator compares the signal power withthe threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described in details inconjunction with the accompanying drawings.

FIG. 1 is a schematic block diagram showing a general structure of areceiver in accordance with the present invention;

FIG. 2 is a flow chart generally illustrating a process of an embodimentin accordance with the present invention;

FIG. 3 shows relationships among signal strength, integration period andfix state according to prior art;

FIG. 4 shows relationships among signal strength, integration period andfix state according to the present invention;

FIG. 5 is a flow chart generally illustrating a process of anotherembodiment in accordance with the present invention;

FIG. 6 is a schematic diagram showing prediction for Doppler frequency;

FIG. 7(A) shows a curve of Doppler frequency of conventional receiver,and FIG. 7(B) shows a curve of Doppler frequency of the presentinvention; and

FIG. 8 is a schematic block diagram showing a Doppler frequencyprocessing device in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram showing a general structure of areceiver in accordance with the present invention. The receiver is usedfor GNSS (e.g. GPS). The receiver includes an antenna 10 for receivingRF signals such as satellite signals of CDMA form, an IF down converter14 for converting the RF signals into IF signals, and a correlation unitfor conducting correlation to the IF signals. The receiver furtherincludes a coherent integration block 20, which has an adder 22 and acoherent integration RAM (or memory of another type) 24, a magnitudecalculator 30, a non-coherent integration block 40, which has an adder32 and a non-coherent integration RAM (or memory of another type), acontroller and a comparator 60. The details will be further describedlater.

An embodiment of the present invention will be described in conjunctionwith FIG. 2, which is flow chart illustrating a process in accordancewith the present invention. In the present embodiment, coherentintegration period (simply referred to as “coherent period”) is set as 1ms. The process starts from step S21, the receiver receives GPS signals,for example. In step S23, coherent integration is conducted to thereceived signals. When a coherent period ends (step S25), the comparator60 compares the signal power obtained from the coherent integration witha threshold (step S27). It is noted that the signal power is calculatedby the magnitude calculator 30, which may calculate the signal powerfrom I and Q components of the signal or simply I component stored inthe coherent integration memory 24, depending on the condition. If thesignal strength is very strong, it is possible to obtain sufficientsignal power within one ms. If the signal power has exceeded thethreshold, the comparator 60 informs the controller 50 (step S33), andthe controller 50 clears up the coherent integration memory 24accordingly (step S35). A new coherent integration can be started then.If the signal power has not exceeded the threshold, the calculatedcoherent integration (i.e. coherent signal power) is sent to thenon-coherent block 40 and stored in the non-coherent integration memory34 to be accumulated, this is called non-coherent integration (stepS29). In the present embodiment, before a predetermined non-coherentmaximum period is reached (step S31), whenever a coherent integration iscompleted, the signal power is accumulated to the non-coherentintegration memory 34. The accumulated signal power is compared with thethreshold by the comparator 60. Once the threshold is achieved, thecontroller passes the signal power to be used for navigation. In otherwords, the non-coherent integration period is dynamically variablerather than fixed.

In a condition that the signal strength is strong, the non-coherentintegration period can be very short. However, in a condition that thesignal strength is weak or signal is blocked, the non-coherentintegration period can be extended. It is noted that the threshold isalso adjusted by the controller 50 as the non-coherent integrationperiod is varied. When the non-coherent integration period is long, theeffect of noises will be more significant, therefore the thresholdshould be higher. The threshold is determined depending on the coherentintegration period and the non-coherent integration period. Theadvantage of the present invention can be easily observed by comparingFIG. 3 and FIG. 4, which will be described as follows.

FIG. 3 shows relationships among signal strength, integration period andfix state according to prior art. In this example, the normal signalstrength is −130 dBm. The signal strength drops below −145 dBm when thereceiver, which may be carried in a vehicle, passes through someblockings such as buildings or trees. When the blockings have beenpassed, the signal strength recovers to be −130 dBm. In prior art, thenon-coherent integration period is fixed, and therefore the integratedsignal power is abruptly drops as shown. The fix state of 3D or 2D fixmode for the receiver is very poor. FIG. 4 shows relationships amongsignal strength, integration period and fix state according to thepresent invention. As can be seen, when the signal strength drops to thelow level, the non-coherent integration period is extended, so that thesignal power can be maintained sufficiently high. Therefore, the fixstate of 3D or 2D fix mode for the receiver can be maintained to beexcellent even when the signal is temporarily blocked.

As mentioned, the coherent integration period is usually ranged from 1ms to 20 ms for GPS in practice. If the coherent integration period isselected to be long, (e.g. 20 ms), the coherent integration period isalso controlled in accordance with the present invention. Assuming thecoherent integration period is set as 20 ms, conventionally, the I and Qcomponents of a received signal is stored and accumulated in thecoherent integration memory. The magnitude calculator calculates thesignal power every 20 ms and the coherent integration memory is clearedfor the next coherent integration. In the present embodiment of thepresent invention, the I and Q components of the signal are accumulatedto the coherent integration memory 24 within 20 ms the same as before.However, the magnitude calculator 30 calculates the signal power every 1ms, and the comparator 60 compares the calculated signal power. When thesignal power exceeds the threshold provided by the controller 50, thecontroller stops the coherent integration and sends the signal power tosuccessive processing. If sufficiently high signal power is failed to beobtained within the first coherent integration period, then the processwill be the same as that described with reference to FIG .2.

As described, the non-coherent integration period is not lengthenedwithout limits A maximum period limit is set in advance. If sufficientsignal power fails to be obtained after the non-coherent integration hasbeen executed for a long period, it may mean that the signal is lost.The present invention also provides a solution to such a problem. Themethod will be described with reference to FIG. 5. Most steps of theprocess indicated by the flow chart of FIG. 5 are generally similar tothat of FIG. 2, but are described more specifically. The main differenceis that in the present embodiment, the solution for solving the problemthat the signal is failed to be obtained even the non-coherentintegration period is extended to the utmost is provided. The processstarts from step S51. In step S52, the method of the present invention,which can be referred to as an auto AGC (automatic gain control)function, is activated and the maximum non-coherent count, by which theallowable non-coherent integration period is directly derived, MaxInc,is set. In step S53, hardware integrates 20 ms coherent data , since thecoherent integration period is set as 20 ms in this embodiment, and thecoherent data is added to the non-coherent (integration) memory 34. Instep S54, the controller 50 sets a threshold according to a currentnon-coherent count, Noncoh_Cnt. When no coherent integration data issent to the non-coherent memory 34 yet, Noncoh Cnt =0. After the firstcoherent integration data is stored into the non-coherent memory 34,Noncoh Cnt =1. The rest can be deduced accordingly. In step S55, thecomparator 60 compares the value of the signal power stored in thenon-coherent memory 34 with a threshold determined by the controller 50.If the signal power has exceeded the threshold, the process goes to stepS56. In step S56, further processing is executed, such as calculatingDoppler frequency and code phase of the signal. In step S57, thenon-coherent RAM 34 is cleared, and the non-coherent count Noncoh_Cnt iszeroed. A new correlation process can be executed. If the value of thesignal power stored in the non-coherent RAM 34 does not exceed thethreshold, then the non-coherent count Noncoh_Cnt is added by 1 in stepS60. The controller 50 checks if Noncoh_Cnt exceeds the upper limtMaxInc in step S61. If not, the integration can be continued. However,if Noncoh_Cnt exceeds MaxInc, it means that the non-coherent integrationperiod has been over a predetermined allowable period. The non-coherentintegration should be abandoned. According to the present embodiment,the receiver uses Doppler frequency and/or code phase prediction schemeto estimate the signal. Taking the Doppler frequency as an example, thereceiver obtains a Doppler frequency curve from the signal having beenreceived. Then a slope of the Doppler frequency curve is calculated. Thereceiver then predicts the curve trend based on the slope.

FIG. 6 is a schematic diagram showing prediction for Doppler frequency.In the drawing, f_(d)+f_(d′) is the Doppler frequency due to themovements of the satellite and the receiver, wherein fd results fromsatellite movement and fd′ results from receiver movement. The term fd′can be obtained by calculating the following equation:

$f_{d}^{\prime} = {\frac{f_{c}}{c}{v(t)}\cos\;\theta}$wherein fc is carrier frequency (e.g. fc of L1 band is 1575.42 MHz); cis the light speed (3×10⁸ m/s), θ is the elevation angle of thesatellite. As can be seen, the variation of Doppler frequency can beestimated by observing the receiver speed v(t) and the satelliteelevation angle θ. For example, if acceleration of the receiver is 1m/s², and θ=0, then the variation rate of Doppler frequency can becalculated as 5 Hz/sec.

FIGS. 7(A) and (B) respectively show the Doppler frequency curveswithout and with use of prediction scheme when the receiver loses lockfor the signal. As can been seen, by considering the slope of theprevious Doppler frequency curve to predict, the section of the curvewhere the signal is lost lock can be estimated.

Another estimation scheme for Doppler frequency curve is free running.FIG. 8 shows a phase lock loop (PLL), which is a Doppler frequencyprocessing device utilized in the receiver. The PLL includes a phasedetector 72 detecting phase of the signal, and a loop filter 74 feedingthe detected phase back to the phase detector 72 via a numeralcontrolled oscillator (NCO) 76. If a value controlling the NCO 76 ismaintained unchanged regardless of the output of the loop filter 74, thephase curve will keep going with the same slope. It is called freerunning. When the signal is lost lock, the value controlling the NCO 76is fixed to the previous value, and so that the phase curve will begenerated continuously with the previous slope. By doing so, the signalcan be simulated when the receiver loses lock for the signal.

While the preferred embodiment of the present invention has beenillustrated and described in details, various modifications andalterations can be made by persons skilled in this art. The embodimentof the present invention is therefore described in an illustrative butnot in a restrictive sense. It is intended that the present inventionshould not be limited to the particular forms as illustrated, and thatall modifications and alterations which maintain the spirit and realm ofthe present invention are within the scope as defined in the appendedclaims.

1. A method for controlling integration of a CDMA signal, said methodimplemented in a receiver and comprising steps of: executing a coherentintegration for a coherent integration period and storing a coherentintegration result into a first memory; calculating a signal power fromthe coherent integration result; accumulating the signal power of eachcoherent integration into a second memory as a non-coherent integrationfor a non-coherent integration period; comparing the accumulated signalpower with a threshold; and repeating the above steps if the accumulatedsignal power has not exceeded the threshold while stopping accumulatingif the accumulated signal power exceeds the threshold; wherein thethreshold is variable and is adjusted depending on at least one of thenon-coherent integration period and the coherent integration period. 2.The method of claim 1, further comprising clearing up the second memoryif the accumulated signal power exceeds the threshold.
 3. The method ofclaim 1, wherein the coherent integration period is divided into aplurality of sub-units, the coherent integration result is calculated toobtain an updated signal power by every sub-unit, the signal power iscompared with the threshold, and the signal power is used in furtherprocessing if it exceeds the threshold.
 4. The method of claim 1,further comprising determining whether a the non-coherent integrationperiod has been over a predetermined limit.
 5. The method of claim 4,wherein the non-coherent integration is abandoned if the non-coherentintegration period exceeds the limit.
 6. The method of claim 4, whereinthe signal is estimated by predicting a Doppler frequency curve with aslope of a portion of the curve which has been generated if thenon-coherent integration period exceeds the predetermined limit.
 7. Themethod of claim 4, wherein the signal is estimated by instructingDoppler frequency processing to switch to free running if thenon-coherent integration period exceeds the predetermined limit.
 8. Areceiver for receiving CDMA signals, said receiver comprising: acorrelator for executing correlation to the signal; a coherentintegration block executing a coherent integration to the signal for acoherent integration period; a magnitude calculator calculating a signalpower from a result of the coherent integration of the coherentintegration block; a non-coherent integration block accumulating thesignal power of each coherent integration for a non-coherent integrationperiod as a non-coherent integration; a comparator comparing the signalpower with a threshold whenever signal power of a coherent integrationis accumulated to the non-coherent block ; and a controller stopping thenon-coherent integration and passing the signal to further process whenthe signal power exceeds the threshold; wherein the threshold isvariable and is adjusted depending on at least one of the non -coherentintegration period and the coherent integration period.
 9. The receiverof claim 8, wherein the coherent integration block has a coherentintegration memory for storing the result of coherent integration, thecoherent integration memory is cleared up whenever a coherentintegration period ends.
 10. The receiver of claim 8, wherein thenon-coherent integration block has a non-coherent integration memory forstoring the accumulated signal power.
 11. The receiver of claim 10,wherein the controller clears up the non-coherent integration memorywhen the signal power exceeds the threshold.
 12. The receiver of claim8, wherein the controller abandons the non-coherent integration if thenon-coherent integration period over a predetermined limit.
 13. Thereceiver of claim 8, wherein the controller estimates the signal bypredicting a Doppler frequency curve with a slope of a portion of thecurve which has been generated if the non-coherent integration periodexceeds a predetermined limit
 14. The receiver of claim 8, furthercomprising a Doppler frequency processing device, the controllerestimates the signal by instructing Doppler frequency processing toswitch to free running if the non-coherent integration period over apredetermined limit.
 15. The receiver of claim 8, wherein the coherentintegration period is divided into a plurality of sub-units, themagnitude calculator calculates the coherent integration result andupdates the signal power by each sub-unit, and the comparator comparesthe signal power with the threshold.
 16. The receiver of claim 15,wherein the coherent integration result is cleared if the signal powerexceeds the threshold.